A prior art device of the kind just described is disclosed in U.S. Pat. No. 5,032,803 issued Jul. 16, 1991 in the name of Michael J. Koch for "Directional Stripline Structure and Manufacture" and assigned to the assignee hereof (the "Koch"patent). That device comprises an inner dielectric board flanked by, and laminated with, two outer dielectric boards to form a multi-layer structure. The two signal conductor means respective to the mentioned pair of transmission lines take the form of two metallized striplines disposed on opposite sides of the inner board to be in broadside coupled relation with each other through the board. The two ground conductor means take the form of ground planes provided by two layers of copper respective to, and covering most of the surfaces of, the outer sides of the two outer boards. The striplines on the inner board are electrically connected through plated through-holes in the outer boards to contact pad lands formed on the outside of those boards.
While the technical performance of the coupler device of the Koch patent has been satisfactory, it has the disadvantage that, because of its multilayer structure, it is wasteful of materials and, moreover, is unduly difficult and expensive to manufacture.
U.S. Pat. No. 4,882,555 issued Nov. 21, 1989 in the name of M. N. Wong for "Plural Plane Waveguide Coupler" discloses a microwave coupler comprising a circuit board in the shape of a square to have four peripheral margins. The board comprises a dielectric substrate having top and bottom surfaces, and top and bottom metallic pads centrally located on, respectively, those top and bottom surfaces and disposed opposite each other through the board to be broadside coupled with each other. Two top metallic strips extend from junctions thereof with opposite ends of the top pad to two respective ports at the top and bottom margins of the board. Two bottom metallic strips extend from junctions thereof with opposite ends of the bottom strip to two respective ports at the left and right margins of the board. The pad and strips on each of the top and bottom surfaces of the substrate are bordered by slots by which they are spaced from expanses of metallic sheets on those surfaces and covering the entire areas thereof outside of said slots.
The Wong coupler has the disadvantage that its four ports are each at a different one of the four peripheral margins of Wong's circuit board so as to make it difficult and expensive to couple the Wong coupler to external microwave circuitry. Another disadvantage of the Wong coupler is that it is wasteful of space.